1. Field of the Invention
This invention generally relates to wireless communication antennas and, more particularly, to a system and method for selectively matching an antenna to selected sub-bands in a communication band.
2. Description of the Related Art
The size of portable wireless communications devices, such as telephones, continues to shrink, even as more functionality is added. As a result, the designers must increase the performance of components or device subsystems and reduce their size, while packaging these components in inconvenient locations. One such critical component is the wireless communications antenna. This antenna may be connected to a telephone transceiver, for example, or a global positioning system (GPS) receiver.
State-of-the-art wireless telephones are expected to operate in a number of different communication bands. In the US, the cellular band (AMPS), at around 850 megahertz (MHz), and the PCS (Personal Communication System) band and DCS band, at around 1900 MHz, are used. Other communication bands include the PCN (Personal Communication Network) at approximately 1800 MHz, the GSM system (Groupe Speciale Mobile) at approximately 900 MHz and 1830 MHz, and the JDC (Japanese Digital Cellular) at approximately 800 and 1500 MHz. Other bands of interest are GPS signals at approximately 1575 MHz, Bluetooth at approximately 2400 MHz, and wideband code division multiple access (WCDMA) at approximately 1850 to 2200 MHz.
Wireless communications devices are known to use simple cylindrical coil or whip antennas as either the primary or secondary communication antennas. Inverted-F antennas are also popular. The resonance frequency of an antenna is responsive to its electrical length, which forms a portion of the operating frequency wavelength. The electrical length of a wireless device antenna is often a multiple of a quarter-wavelength, such as 5λ/4, 3λ/4, λ/2, or λ/4, where λ is the wavelength of the operating frequency, and the effective wavelength is responsive to the physical length of the antenna radiator and the proximate dielectric constant.
Conventionally, each wireless device transceiver (receiver and/or transmitter) is connected to a discrete antenna that resonates in a particular communication band. While the antenna may resonant fairly well across an entire communication band, it cannot be optimally tuned for every channel in the communication band. Thus, the wide-band tuning comes at the expense of optimal efficiency.
Many state-of-the-art wireless devices operate in a number of communication bands. However, it is difficult to locate discrete antennas in a portable wireless device when the device incorporates a number of transceivers, each operating in a different communication band, or one transceiver that can be tuned to operate in a number of communications bands. A brute-force approach has been to add a different resonator or antenna for each communication band. For example, it is known to stack two microstrip patches with different areas to create non-harmonically related resonant frequency responses. Such a design may be inadequate to cover all the required frequencies (communication bands), however. One work-around solution for the above-mentioned antenna has been to widen the bandpass response of the higher communication band, to cover GPS and PCS communications for example, and to use the lower communication band to resonate at cellular band (AMPS) frequencies. However, the widening of the higher band, to improve GPS and PCS performance, comes at the expense of cellular band performance.
Conventional antenna designs incorporate the use of a dielectric material. Generally speaking, a portion of the field that is generated by the antenna returns to the counterpoise (ground), from the radiator, through the dielectric. The antenna is tuned to be resonant at frequencies, and the wavelength of the radiator and dielectric constant have an optimal relationship at the resonant frequency. The most common dielectric is air, with a dielectric constant of 1. The dielectric constants of other materials are defined with respect to air.
Ferroelectric materials have a dielectric constant that changes in response to an applied voltage. Because of their variable dielectric constant, ferroelectric materials are good candidates for making tunable components. Conventional measurement techniques, however, have characterized ferroelectric components as substantially lossy, regardless of the processing, doping or other fabrication techniques used to improve their loss properties. Ferroelectric tunable components operating in RF or microwave regions had been perceived as being particularly lossy. This observation is supported by experience in radar applications where, for example, high radio frequency (RF) or microwave loss is the conventional rule for bulk (thickness greater than about 1.0 mm) FE (ferroelectric) materials especially when maximum tuning is desired. In general, most FE materials are lossy unless steps are taken to improve (reduce) their loss. Such steps include, but are not limited to: (1) pre and post deposition annealing or both to compensate for O2 vacancies, (2) use of buffer layers to reduce surfaces stresses, (3) alloying or buffering with other materials and (4) selective doping.
As demand for the limited range tuning of lower power components has increased in recent years, the interest in ferroelectric materials has turned to the use of thin film rather than bulk materials. The assumption of high ferroelectric loss, however, has carried over into thin film work as well. Conventional broadband measurement techniques have bolstered the assumption that tunable ferroelectric components, whether bulk or thin film, have substantial loss. In wireless communication matching circuits, for example, a Q of greater than 40, and preferably greater than 180 and, more preferably, greater than 350, is necessary at frequencies of about 2 GHz. These same assumptions apply to the design of antenna interface circuitry and transceivers.
Tunable ferroelectric components, especially those using thin films, can be employed in a wide variety of frequency agile circuits. Tunable components are desirable because they permit circuitry to be tuned to selectable channels or sub-bands in a communication band. A tunable component that covers multiple sub-bands potentially reduces the total number of components in a device, as discrete band fixed-frequency components and their associated switches become unnecessary. These advantages are particularly important in wireless handset design, where the need for increased functionality, and lower cost and size are seemingly contradictory requirements. With CDMA handsets, for example, the performance of individual components is highly stressed. FE materials may also permit integration of RF components that, to-date, have resisted shrinkage.
Tunable antenna designs have been disclosed in the Related Applications listed above, and optimal sub-band resonance is possible using an FE dielectric antenna. However, tunable antennas are relatively complex, and more expensive to build than conventional fixed-frequency antennas.
It would be advantageous if an antenna could be optimally matched for particular selected channels in a communication band.
It would be advantageous if the above-mentioned channel-matched antenna had a fixed impedance. That is, if the channel selectively could be performed using a non-tunable antenna.
It would be advantageous if the above-mentioned channel selectablity could be obtained by using an antenna matching circuit that is tunable.